If rolling element bearings are to operate reliably, they must be adequately lubricated to prevent direct contact between the rolling elements, raceways and cage (if present). Loss of lubrication function results in friction and wear, and will quickly lead to bearing failure. Most rolling element bearings (80%) are lubricated with grease, which comprises a base oil, such as a mineral oil, and a thickener, such as a metallic soap. In a grease-lubricated bearing, oil released from the grease forms a thin film that separates the contact between rolling element and bearing raceways; the thickener plays little role in film formation. Replenishment of the oil film is vital, and the end of grease life occurs when this mechanism can no longer take place. In practice, a grease generally reaches the end of its life after many thousands of operating hours.
For obvious reasons, it is important to have an estimate of grease life, so that the grease in a bearing may be replaced before failure is likely. Relubrication interval is therefore a common term to express grease life. Currently, the estimation of relubrication intervals is based on empirical models constructed from statistical interpretations of test data. A number of test methods exist. These include the SKF R2F Test (DIN 51806), the SKF ROF Test, the FAG FE8 Test (DIN 51821) and the FAG FE9 Test. In these tests, a population of bearings lubricated with a particular grease is tested until the grease fails.
This is deemed to occur when e.g. a measured temperature exceeds a predefined maximum value (SKF ROF) or a measured torque exceeds a predefined maximum value (FAG FE8).
The aforementioned tests are laboratory performance tests, which are generally conducted under artificially severe conditions, i.e. high temperature and/or high speed and/or high load. The severe conditions are necessary in order to reduce the time required to evaluate a grease's performance, and thereby reduce testing costs. A typical duration for a grease life test is between 500 and 1500 hours, which is a mere fraction of the actual grease life of a real grease-lubricated bearing operating under normal conditions.
It must therefore be recognized that although valuable data is obtained from conventional testing methods, the severity of the conditions under which they are performed means that the tests have only a limited ability to predict the suitability of a lubricating grease for long-term use under normal conditions. Moreover, conventional grease testing methods only provide statistical data, which is then used to predict the relubrication interval for a statistical grease-lubricated bearing. No prediction can be made about the relubrication interval for a specific individual bearing.
Consequently, there is room for improvement.